Method for enhanced fracture cleanup using redox treatment

ABSTRACT

A method for improved hydrocarbon recovery from a formation due to cleanup of a residual viscous material is provided. The method comprising the steps of fracturing the formation with a fracturing fluid to generate fractures, the fracturing fluid comprising a viscous fluid component operable to fracture the formation leaving behind residual viscous material in the fractures, the viscous fluid having a viscosity; a proppant component comprising a proppant, the proppant operable to hold open the fractures, wherein the proppant component is carried to the fractures by the viscous fluid component; and a cleanup fluid, the cleanup fluid comprising: an acid precursor operable to trigger an exothermic reaction component, and the exothermic reaction component operable to generate heat, where the generated heat is operable to reduce a viscosity of the residual viscous material to create a reduced viscosity material operable to flow from the formation.

PRIORITY

This U.S. non-provisional patent application declares priority to U.S.Provisional Patent Application No. 61/980,664, filed Apr. 17, 2014, theentire disclosure of which is hereby expressly incorporated herein byreference.

FIELD

This disclosure relates to a composition and method to improve therecovery of hydrocarbons from a fractured formation. More specifically,this disclosure relates to a composition and method to reduce theviscosity of a fracturing fluid.

BACKGROUND

Hydraulic fracturing fluids containing proppants are used extensively toenhance productivity from hydrocarbon reservoir formations, includingcarbonate and sandstone formations. During hydraulic fracturingoperations, a fracturing treatment fluid is pumped under a pressure andrate sufficient for cracking the formation of the reservoir and creatinga fracture. Fracturing operations usually consist of three main stagesincluding a pad fluid stage, a proppant fluid stage, and an overflushfluid stage. The pad fluid stage typically consists of pumping a padfluid into the formation. The pad fluid is a viscous gelled fluid whichinitiates and propagates the fractures. Auxiliary fractures canpropagate from the fractures to create fracture networks. A fracturenetwork can comprise fractures and auxiliary fractures. Auxiliaryfractures can connect the fractures.

The proppant fluid stage involves pumping a proppant fluid into thefractures of the formation. The proppant fluid contains proppants mixedwith a viscous gelled fluid or a visco-elastic surfactant fluid. Theproppants in the proppant fluid are lodged in the fractures and createconductive fractures through which hydrocarbons flow. The final stage,the overflush stage, includes pumping a viscous, gelled fluid into thefractures to ensure the proppant fluid is pushed inside the fractures.While the three stages have different aims, all three make use of highlyviscous and/or gelled fluids to achieve those aims.

A downside of the traditional method is that a high volume of gelled orpolymeric materials can be left behind in the fractures. The gelledmaterials can be concentrated around the proppant in the fractures orcan be freely in the fractures. The gelled material acts to block thefractures reducing the fracture conductivity. The hydrocarbons whichflow from the reservoir formation are unable to move the gelledmaterials. Traditional methods for cleaning the fractures involveviscosity breakers or other elements to breakdown the fluid. Thesetraditional methods suffer from an inability to completely cleanup thefractures, leaving residual viscous material and reduced conductivity.

SUMMARY

This disclosure relates to a composition and method to improve therecovery of hydrocarbons from a fractured formation. More specifically,this disclosure relates to a composition and method to reduce theviscosity of a fracturing fluid, such as, for example, a gelled and/orviscous fracturing fluid.

In one aspect, a method for improved hydrocarbon recovery from aformation due to cleanup of a residual viscous material is provided. Themethod includes the step of fracturing the formation with a fracturingfluid to generate fractures. The fracturing fluid includes a viscousfluid component, the viscous fluid component operable to fracture theformation to create fractures leaving behind the residual viscousmaterial in the fractures, the viscous fluid component having aviscosity, a proppant component, the proppant component includes aproppant, the proppant operable to hold open the fractures, wherein theproppant component is carried to the fractures by the viscous fluidcomponent, and a cleanup fluid.

The cleanup fluid includes an acid precursor, the acid precursoroperable to trigger an exothermic reaction component, and the exothermicreaction component operable to generate heat, wherein the heat isoperable to reduce a viscosity of the residual viscous material tocreate a reduced viscosity material, the reduced viscosity materialoperable to flow from the formation. Fractures can include auxiliaryfractures, which propagate from the fractures.

In certain aspects, the exothermic reaction component includes anammonium containing compound and a nitrite containing compound. Incertain aspects of the present disclosure, the ammonium containingcompound is NH₄Cl and the nitrite containing compound is NaNO₂. Incertain aspects of the disclosure, the acid precursor is triacetin.

In a second aspect of the present disclosure, a cleanup fluid forreducing a viscosity of a residual viscous material in fractures isprovided. The cleanup fluid includes an acid precursor, the acidprecursor operable to trigger an exothermic reaction component, and theexothermic reaction component operable to generate heat, wherein theheat is operable to reduce a viscosity of the residual viscous materialto create a reduced viscosity material, the reduced viscosity materialoperable to flow from the fractures.

In certain aspects, the exothermic reaction component includes anammonium containing compound and a nitrite containing compound. Incertain aspects of the present disclosure, the ammonium containingcompound is NH₄Cl and the nitrite containing compound is NaNO₂. Incertain aspects of the present disclosure, the acid precursor istriacetin.

In a third aspect, a method to cleanup fractures post hydraulicfracturing is provided. The method includes the steps of fracturing aformation in a hydraulic fracturing operation to produce fractures, andinjecting a cleanup fluid into the fractures to reduce a viscosity of aresidual viscous material.

In certain aspects of the present disclosure, the step of fracturing theformation includes the step of fracturing the formation with afracturing fluid to generate fractures. The fracturing fluid includes aviscous fluid component, the viscous fluid component operable tofracture the formation to create fractures leaving behind the residualviscous material in the fractures, the viscous fluid component having aviscosity, and a proppant component, the proppant component comprising aproppant, the proppant operable to hold open the fractures, wherein theproppant component is carried to the fractures by the viscous fluidcomponent. In certain aspects of the present disclosure, the cleanupfluid includes an acid precursor, the acid precursor operable to triggeran exothermic reaction component, and the exothermic reaction componentoperable to generate heat, wherein the heat is operable to reduce aviscosity of the residual viscous material to create a reduced viscositymaterial, the reduced viscosity material operable to flow from thefractures. In certain aspects of the present disclosure, the exothermicreaction component includes an ammonium containing compound and anitrite containing compound. In certain aspects, the ammonium containingcompound is NH₄Cl and the nitrite containing compound is NaNO₂. Incertain aspects, the acid precursor is triacetin.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with regard to the followingdescriptions, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of thedisclosure and are therefore not to be considered limiting of thedisclosure's scope as it can admit to other equally effectiveembodiments.

FIG. 1 is a graphic representation of the effect of the cleanup fluid onthe viscosity of the residual viscous material.

FIG. 2 is a graphic representation of the heat and pressure generated bythe exothermic reaction component.

FIGS. 3a and 3b are pictorial representations of the residual viscousmaterial before the reaction of an exothermic reaction component of thecleanup fluid.

FIG. 4 is a graphic representation of the effect of the reaction of theexothermic reaction component on the viscosity of a fracturing fluid.

DETAILED DESCRIPTION

While the disclosure will be described with several embodiments, it isunderstood that one of ordinary skill in the relevant art willappreciate that many examples, variations and alterations to theapparatus and methods described herein are within the scope and spiritof the disclosure. Accordingly, the embodiments described herein are setforth without any loss of generality, and without imposing limitations,on the claims.

In one aspect, a method for improved hydrocarbon recovery from aformation due to cleanup of a residual viscous material is provided. Thehydraulic fracturing operation fractures the formation using fracturingfluid to create fractures. Formations include sandstone and carbonate,for example.

The fracturing fluid includes a viscous fluid component and a proppantcomponent. The viscous fluid component has a viscosity. The viscousfluid component is operable to increase the viscosity of the fracturingfluid. Viscous fluid components include viscosified water-based fluids,non-viscosified water-based fluids, gel-based fluids, gel oil-basedfluids, acid-based fluids, and foam fluids. Gel-based fluids includecellulose derivatives and guar-based fluids. Cellulose derivativesinclude carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxypropyl cellulose, and methyl hydroxylethyl cellulose.

Guar-based fluids include hydroxypropyl guar, carboxymethyl guar, guarcross-linked boron ions from an aqueous borax/boric acid solution, andguar cross-linked with organometallic compounds. Organometalliccompounds include zirconium, chromium, antimony, and titanium salts. Geloil-based fluids include aluminum phosphate-ester oil gels. In at leastone embodiment of the present disclosure, the viscous fluid component isan aqueous guar solution, having a concentration of guar gum betweenabout 0.1% and about 15%, between about 0.1% and about 10%, betweenabout 1% and about 10%, between about 2% and about 8%, and between about4% and about 6%.

The proppant component includes a proppant. The proppant is operable tohold open fractures created by the viscous fluid component. Anyproppants capable of holding open fractures to create a conductivefractures are suitable for use in the present disclosure. In someembodiments, the proppant component includes a viscous carrier fluidhaving a viscosity.

Viscous carrier fluids include viscosified water-based fluids,non-viscosified water-based fluids, gel-based fluids, gel oil-basedfluids, acid-based fluids, and foam fluids. Gel-based fluids includecellulose derivatives and guar-based fluids. Cellulose derivativesinclude carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxypropyl cellulose, and methyl hydroxylethyl cellulose.

Guar-based fluids include hydroxypropyl guar, carboxymethyl guar, guarcross-linked boron ions from an aqueous borax/boric acid solution, andguar cross-linked with organometallic compounds. Organometalliccompounds include zirconium, chromium, antimony, and titanium salts. Geloil-based fluids include aluminum phosphate-ester oil gels. In someembodiments, the hydraulic fracturing operation uses a one stagefracturing fluid, in which the fracturing fluid includes both theviscous fluid component and the proppant component, in which the viscousfluid component carries the proppant component to the fractures.

In at least one embodiment, the hydraulic fracturing operation uses amulti-stage fracturing fluid in which the viscous fluid component isinjected into the formation, followed by the proppant component in theviscous carrier fluid. In some embodiments, the injection of theproppant component is followed by injection of additional viscous fluidsto ensure the proppants are placed in the fractures. The additionalviscous fluids have a viscosity.

In some embodiments, the viscosity of the viscous fluid component, theviscous carrier fluid, and additional viscous fluids are the same. Insome embodiments, the viscosity of the viscous fluid component, theviscous carrier fluid, and additional viscous fluids are different. Theinjection of the fracturing fluid ceases after the proppants are placedin the fractures and the fracturing fluid is allowed to seep from thefractures. In some embodiments, the injection of the hydraulicfracturing fluid including the viscous fluid component and/or theproppant component and/or the overflush component and/or the exothermicreaction component does not generate foam or introduce foam into thehydraulic formation including the hydraulic fractures.

The hydraulic fracturing operation can leave residual viscous materialin the fractures of a hydraulic formation. Residual viscous materialscan include carboxymethyl cellulose, hydroxyethyl cellulose,carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methylhydroxyl ethyl cellulose, guar gum, hydroxypropyl guar, carboxymethylguar, guar cross-linked with boron, aluminum phosphate-ester oil gel,and guar cross-linked with organometallic compounds. Organometalliccompounds include zirconium, chromium, antimony, and titanium salts. Insome embodiments of the present disclosure, the residual viscousmaterial is a gelled material. In some embodiments of the presentdisclosure, the residual viscous material is a polymeric material. In atleast one embodiment of the present disclosure, the residual viscousmaterial is guar gum. The residual viscous material has a viscositygreater than the fracturing fluid. In at least one embodiment of thepresent disclosure, the residual viscous material is surrounding and/oradjacent to the proppants placed in the fractures.

The cleanup fluid acts, after the proppants have been placed in thefractures, to remove the residual viscous material. In one embodiment,the cleanup fluid is mixed with the fracturing fluid. In at least oneembodiment of the present disclosure, where a multi-stage fracturingfluid is used, the cleanup fluid is a component of the fluids used ateach stage of the hydraulic fracturing operation. In an alternateembodiment, the cleanup fluid is added only to the fluid of the finalstage of the hydraulic fracturing operation, such as, for example, theoverflush stage. In some embodiments, the cleanup fluid is pumped to thefractured formation as a separate step following the hydraulicfracturing operation.

In some embodiments, the cleanup fluid includes an acid precursor and anexothermic reaction component. The reaction of the exothermic reactioncomponent results in a release of kinetic energy and thermal energy. Thereaction of the exothermic reaction component generates heat andincreases the pressure. The generated heat increases the temperature ofthe surrounding fluids, including fracturing fluid remaining in thefractures and residual viscous material. The increase in temperaturereduces the viscosity of the fracturing fluid. The increase intemperature reduces the viscosity of the residual viscous material leftin the fractures to create a reduced viscosity material. The reducedviscosity material flows from the fractures of the formation to thewellbore. The increase in pressure provides lift energy to push thereduced viscosity materials through the wellbore toward the surface. Theremoval of the residual viscous material increases the conductivity ofthe fractures. Increased conductivity of the fractures increases seepageof the fracturing fluid, improves fracturing efficiency, minimizes needfor additional fracturing jobs, minimizes time between fracturing andwell production, and increases hydrocarbon flow, which translates toincreased hydrocarbon recovery.

The acid precursor is any acid that releases hydrogen ions to triggerthe reaction of the exothermic reaction component. Acid precursorsinclude triacetin (1,2,3-triacetoxypropane), methyl acetate, HCl, andacetic acid. In at least one embodiment, the acid precursor istriacetin. In at least one embodiment, the acid precursor is aceticacid.

The exothermic reaction component includes one or more redox reactantsthat exothermically react to produce heat and increase pressure.Exothermic reaction components include urea, sodium hypochlorite,ammonium containing compounds, and nitrite containing compounds. In atleast one embodiment of the present disclosure, the exothermic reactioncomponent includes ammonium containing compounds. Ammonium containingcompounds include ammonium chloride, ammonium bromide, ammonium nitrate,ammonium sulfate, ammonium carbonate, and ammonium hydroxide.

In at least one embodiment, the exothermic reaction component includesnitrite containing compounds. Nitrite containing compounds includesodium nitrite and potassium nitrite. In at least one embodiment, theexothermic reaction component includes both ammonium containingcompounds and nitrite containing compounds. In at least one embodiment,the ammonium containing compound is ammonium chloride, NH₄Cl. In atleast one embodiment, the nitrite containing compound is sodium nitrite,NaNO₂.

In at least one embodiment of the present disclosure, the exothermicreaction component includes two redox reactants: NH₄Cl and NaNO₂, whichreact according to the following:

In a reaction of the exothermic reaction components according to theabove equation, generated gas and heat contribute to the reduction ofthe viscosity of the residual viscous material.

The exothermic reaction component is triggered to react. In at least oneembodiment of the present disclosure, the exothermic reaction componentis triggered within the fractures. In at least one embodiment of thepresent disclosure, the acid precursor triggers the exothermic reactioncomponent to react by releasing hydrogen ions.

In at least one embodiment, the exothermic reaction component istriggered by heat. The wellbore temperature is reduced during a pre-padinjection or a pre-flush with brine and reaches a temperature below 120°F. (48.9° C.). The fracturing fluid of the present disclosure is theninjected into the well and the wellbore temperature increases. When thewellbore temperatures reaches a temperature greater than or equal to120° F., the reaction of the redox reactants is triggered. In at leastone embodiment, the reaction of the redox reactants is triggered bytemperature in the absence of the acid precursor. In at least oneembodiment, the exothermic reaction component is triggered by heat whenthe exothermic reaction component is within the fractures.

In at least one embodiment, the exothermic reaction component istriggered by pH. A base is added to the fracturing fluid of the presentdisclosure to adjust the pH to between 9 and 12. In at least oneembodiment, the base is potassium hydroxide. The fracturing fluid withthe base is injected into the formation. Following the injection of thefracturing fluid, an acid is injected to adjust the pH to below 6. Whenthe pH is below 6, the reaction of the redox reactants is triggered. Inat least one embodiment, the exothermic reaction component is triggeredby pH when the exothermic reaction component is within the fractures.

In at least one embodiment of the present disclosure, the cleanup fluidis introduced to the fractures following the hydraulic fracturingoperation. Dual-string coiled tubing is used to introduce the exothermicreaction component and the acid precursor to the wellbore. In at leastone embodiment, the exothermic reaction component includes NH₄Cl andNaNO₂. The acid precursor is acetic acid. The acetic acid is mixed withNH₄Cl and injected in parallel with the NaNO₂, using different sides ofthe dual-string coiled tubing. The exothermic reaction component and theacid precursor mix within the fractures.

EXAMPLES Example 1

An exothermic reaction component of a cleanup fluid consisting of 3MNH₄Cl and 3M NaNO₂ was added to a solution of 1% by volume guar at roomtemperature, see FIG. 3. The exothermic reaction component was triggeredby heat. The viscosity of the solution was measured before, during, andafter the reaction using a Chandler viscometer. Prior to reaction of theexothermic reaction component, the viscosity of the residual viscousmaterial was 85 cP. FIG. 1 is a graph of the viscosity following thereaction of the exothermic reaction component. The graph shows that theviscosity of the residual viscous material was reduced to less than 8.5cP. FIG. 3b shows the solution, including the residual viscous materialafter the reaction of the exothermic reaction component.

Example 2

A solution of an exothermic reaction component was prepared from 3MNH₄Cl and 3M NaNO₂. The solution was placed in an autoclave reactor atroom temperature and an initial pressure of 1,000 psi. The reactortemperature was increased. The reaction was triggered at about 120° F.,see FIG. 2. Due to the reaction, the temperature in the reactor reacheda temperature of 545° F. and a pressure of 3,378 psi, see FIG. 2.

Example 3

The exothermic reaction component showed compatibility with the viscousfluid component (here an x-linked gel). The fracturing fluid with theviscous fluid component, the exothermic reaction component, and theproppant component was also prepared and showed compatibility. Thefracturing fluid, without the proppant component, was activated in theautoclave reactor by heating to the wellbore temperature to trigger thereaction of the exothermic reaction component. The heat generated by thereaction reduced the viscosity of the viscous fluid component to producea reduced viscosity material, without injecting the viscosity breaker.Using a chandler viscometer, the viscosity of the fracturing fluid,containing the viscous fluid component and the exothermic reactioncomponent, was measured pre-reaction and post-reaction. The viscosity ofthe fracturing fluid was reduced from 1600 cp to 10 cp, as shown in FIG.4. The results show that the exothermic reaction component and this typeof treatment can clean-up the fractures post a fracturing job.

Although the present disclosure has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereupon without departing from the principle and scope of thedisclosure. Accordingly, the scope of the present disclosure should bedetermined by the following claims and their appropriate legalequivalents.

The singular forms “a,” “an,” and “the” include plural referents, unlessthe context clearly dictates otherwise.

Optional or optionally means that the subsequently described event orcircumstances can or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

Throughout this application, where patents or publications arereferenced, the disclosures of these references in their entireties areintended to be incorporated by reference into this application, in orderto more fully describe the state of the art to which the disclosurepertains, except when these references contradict the statements madeherein.

As used herein and in the appended claims, the words “comprise,” “has,”and “include” and all grammatical variations thereof are each intendedto have an open, non-limiting meaning that does not exclude additionalelements or steps.

As used herein, terms such as “first” and “second” are arbitrarilyassigned and are merely intended to differentiate between two or morecomponents of an apparatus. It is to be understood that the words“first” and “second” serve no other purpose and are not part of the nameor description of the component, nor do they necessarily define arelative location or position of the component. Furthermore, it is to beunderstood that that the mere use of the term “first” and “second” doesnot require that there be any “third” component, although thatpossibility is contemplated under the scope of the present disclosure.

What is claimed is:
 1. A method for improved hydrocarbon recovery from aformation due to cleanup of a residual viscous material, the methodcomprising the steps of: fracturing the formation with a fracturingfluid to generate fractures, the fracturing fluid comprising: a viscousfluid component, the viscous fluid component operable to fracture theformation to create the fractures leaving behind the residual viscousmaterial in the fractures, the viscous fluid component having aviscosity; a proppant component, the proppant component operable to holdopen the fractures, wherein the proppant component is carried to thefractures by the viscous fluid component; and after fracturing theformation, reducing a viscosity of the residual viscous material with acleanup fluid, the cleanup fluid comprising: an acid precursor, the acidprecursor operable to trigger an exothermic reaction component, and theacid precursor comprising at least one component selected from the groupconsisting of: triacetin, methyl acetate, hydrochloric acid, and aceticacid; and the exothermic reaction component operable to generate heatafter the proppant component has been placed in the fractures, and theexothermic reaction component comprising at least one component selectedfrom the group consisting of: urea, sodium hypochlorite, ammoniumchloride, ammonium bromide, ammonium nitrate, ammonium sulfate, ammoniumcarbonate, ammonium hydroxide, sodium nitrite, and potassium nitrite,wherein the heat is operable to reduce the viscosity of the residualviscous material to create a reduced viscosity material, the reducedviscosity material operable to flow from the formation, and wherein themethod for improved hydrocarbon recovery generates substantially nofoam.
 2. The method of claim 1, wherein the exothermic reactioncomponent comprises an ammonium containing compound and a nitritecontaining compound.
 3. The method of claim 2, wherein the ammoniumcontaining compound comprises NH₄Cl and the nitrite containing compoundcomprises NaNO₂.
 4. The method of claim 1, wherein the acid precursorcomprises triacetin.
 5. The method of claim 1, wherein the step offracturing the formation with a fracturing fluid to generate fracturesfurther comprises the step of forming auxiliary fractures and a fracturenetwork.
 6. A method to cleanup fractures in hydraulic fracturingoperations, the method comprising the steps of: fracturing a formationin a hydraulic fracturing operation to generate fractures, wherein thestep of fracturing the formation comprises the step of fracturing theformation with a fracturing fluid to generate fractures, the fracturingfluid comprising: a viscous fluid component, the viscous fluid componentoperable to fracture the formation to create the fractures leavingbehind residual viscous material in the fractures, the viscous fluidcomponent having a viscosity; a proppant component, the proppantcomponent operable to hold open the fractures, wherein the proppantcomponent is carried to the fractures by the viscous fluid component;and injecting a cleanup fluid into the fractures to reduce a viscosityof the residual viscous material by generation of heat produced from anexothermic reaction, wherein the exothermic reaction is triggered toreact at least in part by heat of the formation, the cleanup fluidoperable to reduce the viscosity of the residual viscous material afterthe proppant component has been placed in the fractures, wherein themethod to cleanup fractures generates substantially no foam.
 7. Themethod of claim 6, wherein the cleanup fluid comprises: an acidprecursor, the acid precursor operable to trigger an exothermic reactioncomponent, and the acid precursor comprising at least one componentselected from the group consisting of: triacetin, methyl acetate,hydrochloric acid, and acetic acid; and the exothermic reactioncomponent operable to generate heat, and the exothermic reactioncomponent comprising at least one component selected from the groupconsisting of: urea, sodium hypochlorite, ammonium chloride, ammoniumbromide, ammonium nitrate, ammonium sulfate, ammonium carbonate,ammonium hydroxide, sodium nitrite, and potassium nitrite, wherein theheat is operable to reduce the viscosity of the residual viscousmaterial to create a reduced viscosity material, the reduced viscositymaterial operable to flow from the fractures.
 8. The method of claim 7,wherein the exothermic reaction component comprises an ammoniumcontaining compound and a nitrite containing compound.
 9. The method ofclaim 8, wherein the ammonium containing compound comprises NH₄Cl andthe nitrite containing compound comprises NaNO₂.
 10. The method of claim7, wherein the acid precursor comprises triacetin.
 11. The method ofclaim 7, wherein the step of fracturing the formation with a fracturingfluid to generate fractures further comprises the step of formingauxiliary fractures and a fracture network.
 12. The method of claim 7,further comprising the step of triggering the exothermic reactioncomponent with a trigger selected from the group consisting of: arelease of hydrogen ions, an increase in temperature of the exothermicreaction component, and combinations thereof.
 13. The method of claim 6,where the step of fracturing the formation comprises adding a base tothe fracturing fluid to increase pH of the fracturing fluid and theformation.
 14. The method of claim 6, where the step of injecting thecleanup fluid into the fractures to reduce the viscosity of the residualviscous material reduces the viscosity of the residual viscous materialto a reduced viscosity, the reduced viscosity being about 1/10 of theviscosity of the residual viscous material.
 15. The method of claim 6,further comprising the step of triggering an exothermic reactioncomponent in the cleanup fluid by allowing an internal formationtemperature of the formation to reach about 120° F.
 16. The method ofclaim 6, further comprising the steps of: initially raising pH of thefracturing fluid to between about pH 9 and about pH 12, and afterinjecting the cleanup fluid into the fractures, lowering pH of thecleanup fluid to below about 6.